Implement get_arp_table

[kace]

I've had success implementing this method locally and, after writing some tests, want to push this upstream.

[jpsutton]

@kace I see you have a fork of this project in your namespace. Any chance you could commit your changes there and submit a pull request (or a series of them)? At the very least, I'd like to have your changes for my own use. :)